Internet technology is now in widespread use in society, and the early establishment of quantum cryptography communications is desired in order to provide security for electronic commerce or the like on the Internet. The quantum cryptography communications are carried out by bringing one bit of information into correspondence with the polarized state of a photon. When an outsider on a communication channel wiretaps the information, the polarized state of the photon is destroyed and thus the occurrence of wiretapping is detected immediately.
Implementation of quantum cryptographic technology requires an optical semiconductor device such as a single-photon generator for generating a single photon that serves as an information carrier. Some types of single-photon generators until now have been reported, and a device of such a structure shown in FIG. 1 has been proposed in Non-patent Document 1 given below.
FIG. 1 is a cross-sectional view of a conventional single-photon generator.
In the single-photon generator, a p-type contact layer 2 made of p-type GaAs is formed on a GaAs substrate 1 with a buffer layer (not shown) therebetween. A bottom barrier layer 3 made of GaAs, a quantum dot 4 made of InAs, and a top barrier layer 5 made of GaAs are formed in this order on the p-type contact layer 2. Further, an n-type contact layer 6 made of n-type GaAs is forced on the top barrier layer 5, and a n-side electrode layer 7 is formed on the n-type contact layer 6 with ohmic contact.
The layers 3 to 6 are patterned into a mesa shape in cross section. A lead electrode 9 is formed on the side surface of the mesa with an insulating layer 8 therebetween, and the current is supplied to the n-side electrode layer 7 via the lead electrode 9. On the other hand, a p-side electrode layer 10 is formed on the p-type contact layer 2 beside the mesa, and the p-side electrode layer 10 is in ohmic contact with the p-type contact layer 2.
In this single-photon generator, a single photon created by recombination of carriers is generated in the quantum dot 4 by passing the current between the n-side electrode layer 7 and the p-side electrode layer 10, and the single photon 11 thus generated is taken out of the device through a window 7a of the n-type electrode layer 7.
FIG. 2 is a plan view of this single-photon generator. FIG. 1 corresponds to a cross-sectional view taken along the line I-I of FIG. 2.
As shown in FIG. 2, the planar shape of the n-type contact layer 6 is square, and the length of its side D is about 10 μm. A region where the n-type contact layer 6 is in contact with the n-side electrode layer 7 functions as a contact region CR0 through which a current is injected into the quantum dot 4 (see FIG. 1). In an example shown in FIG. 2, the n-side electrode layer 7 covers nearly the entire area of the n-type contact layer 6, and therefore the contact region CR0 has a square shape whose side is 10 μm, which is substantially the same as a region where the n-type contact layer 6 is formed.
Incidentally, if the contact region CR0 shown in FIG. 2 has a large area, power consumption increases due to the excessive current injection into the quantum dot 4 (see FIG. 1). The excessive current leads to the generation of heat in the quantum dot 4, which in turn causes the shift of the wavelength of the single photon than designed value. Moreover, the large area of the contact region CR0 leads to large parasitic capacitance between the n-side electrode layer 7 and the GaAs substrate 1. Therefore, when the frequency of a signal voltage to the electrode layers 7 and 10 is increased, signal delay caused by the parasitic capacitance increases. It is therefore desirable that the contact region CR0 have as small an area as possible.
Incidentally, examples of technologies related to the present invention are also disclosed in Patent Documents 1 and 2 given below.
Non-patent Document 1: Zhiliang Yuan et al., Science 295, 102 (2002)
Patent Document 2: Japanese Unexamined Patent Application Laid-open Publication No. 2004-253657
Patent Document 3: Japanese Unexamined Patent Application Laid-open Publication No. Hei 4-61176